The photochemical oxidation of di-n-butyl phthalate (DBP) by •OH radicals from nitrous acid (HONO) in atmospheric hydrometeors was explored by two techniques, steady-state irradiation, and laser flash photolysis (LFP). The effects of atmospheric liquid parameters on DBP transformation were systematically evaluated, showing that DBP does not react with HONO directly and •OH-initiated reactions are crucial steps for consumption and transformation of DBP. Two reaction channels are operative: •OH addition and hydrogen atom abstraction. The overall rate constant for the reaction of DBP with •OH is 5.7 × 109 M-1 s-1, and its specific rate constant for addition is 3.7 × 109 M-1 s-1 determined by using laser flash photolysis technique. Comparing the individual reaction rate constant for aromatic ring addition with the total rate constant, the majority of the •OH radicals (about 65%) attack the aromatic ring. The major transformation products were identified by GC-MS, and the trends of their yields derived from both ring addition and H-abstraction with time are discussed. These results provide important insights into the photochemical transformation of DBP in atmospheric hydrometeors and contribute to atmospheric aerosol chemistry.